It is known that certain metal alloys and particularly certain titanium alloys, such as Ti-6Al-4V, which are widely used in the Aerospace industry, exhibit superplastic behavior. Superplasticity is the capability of a material to undergo unusually large elongations before necking, while maintained within given ranges of temperature and strain rate.
As by way of specific examples, Ti-6Al-4V alloy demonstrates superplastic behavior and can be made to undergo elongations greater than 100% when maintained at temperatures within the range of about 1500.degree.-1800.degree. F. and subjected to strain rates in the range of about 10.sup.-6 to 10.sup.-3 inch/inch/second.
Presently known commercial apparatus adapted for use in effecting superplastic forming of titanium alloy sheets typically employ differential pressures within an inert gas environment to effect mold forming of sheets against a female mold. Apparatus of this type, which is disclosed for instance in U.S. Pat. No. 3,934,441, is expensive to construct and operate and has the additional disadvantage of requiring the use of mechanical seals to maintain the gas environment, which act to prevent/interfere with conventional draw forming of the sheet material incident to the superplastic forming operation.
The utilization of gas or liquid pressure mediums in superplastic forming apparatus is also disclosed, as by way of further example, by U.S. Pat. Nos. 3,529,458 and 3,898,827.
It has also been proposed in U.S. Pat. No. 3,768,142 to employ a heat expandable polymeric foam material to apply forming pressure to a zinc-aluminum alloy sheet subjected to a superplastic forming temperature on the order of about 550.degree. F.; and in U.S. Pat. No. 3,605,477 to form titanium alloy blanks by contact with mated heated forming tools after the blanks have been raised to forming temperature in a separate heating unit. These proposals possess certain drawbacks overcome by the above mentioned inert gas forming operation and/or are not susceptible for use for all known superplastic materials.
Lastly, Applicant has been appraised of U.S. Pat. No. 699,018, which is directed towards an apparatus for forming dental plates, wherein sand is employed as the forming or pressure transmitting medium. In the use of this apparatus, a metal plate is laid on a die disposed at the bottom of a cylindrical forming chamber and the chamber subsequently filled with a charge of sand. A mechanically coupled plunger-sleeve device is then inserted into the upper end of the chamber and the charge of sand compacted to effect initial forming of the metal plate by means of hammer blows applied to the plunger, The forming operation is completed by replacing the original plunger successively with one or more additional plungers, which are freely movable relative to the sleeve, and applying hammer blows to the additional plunger(s) until the metal plate is given its final form. The metal plate may be heated externally of the apparatus.